A plasma display panel (PDP) and a liquid crystal display (LCD), which are typical flat or thin type display devices, do not use an electron gun as used in a cathode-ray tube display. Instead of this, because they use thin film transistors (TFTs) as a switching element to illuminate each pixel, a substantial reduction in thickness of the device has been realized. An element using amorphous silicon or polycrystalline silicon in the channel is used in the TFT.
Organic EL (Electro Luminescence) devices are noted as an element for a next generation display in which disadvantages in the expected life span, contrast, responsiveness, power consumption, etc. possessed by the PDP and the LCD can be improved, and research to put the organic EL device to practical use has been in progress. Further, the organic EL device is a thin film device of organic materials, and still further reduction in thickness than the PDP and the LCD is in progress. Furthermore, because the organic materials, having a light weight and flexibility, are used as raw materials, realization of a wall-mounted display and a flexible display is considered to be possible. In order to make use of such advantages, research has been performed actively to make a TFT with the organic materials having a light weight and flexibility, for driving an organic EL device.
Because the organic materials are dissolved in an organic solvents, etc. and can be handled at around room temperature, it is expected that the manufacture of the organic TFTs also uses the solution and it can be manufactured with a process of coating and printing. In the manufacture by coating and printing, vacuum equipment, thermal processes at high temperature, etc., which are essential in a silicon device, are not necessary, and a substantial reduction in manufacturing cost can be realized. On the other hand, the performance of the organic TFT as a transistor is substantially inferior compared to the silicon device, and it has not come to practical use in the present condition.
Organic molecules are roughly classified into organic molecules with a small molecular weight (a low molecule) such as a monomer and an oligomer, and organic molecules with a large molecular weight categorized as a polymer, and the organic TFTs are also classified into two kinds depending on with which organic molecules the channel is formed. In the organic TFTs using low molecules in the channel, it is proved that mobility of the carriers flowing in the channel can be increased to at least the same level as amorphous silicon if crystallinity of the organic molecular crystal can be kept favorably, and it has an advantage that the TFT with a high operation speed is easily obtained. However, the method of forming which uses vacuum deposition of organic molecules is a general method for the formation of the channel, and has a disadvantage that it is difficult to reduce the manufacturing cost. On the other hand, because the organic TFTs using polymers in the channel are easy to apply a coating and a printing process in the manufacture, the manufacturing cost can be reduced. However, the mobility of the carriers flowing in the channel that has been realized is only about 1/10 at most compared to the low molecule organic TFTs using low molecules in the channel, and it has a disadvantage that the performances of the TFTs are low.
In the organic TFT, there is a problem that the operation speed of the TFT is low compared to a silicon-based TFT in general. This is because the mobility of the carriers flowing in the channel is small, and scattering of the carriers in the channel is known to be one of the big causes. In order to reduce the scattering of the carriers, making the crystal grains of the crystals forming the channel large and reducing the number of crystal grain boundaries that the carriers pass when contacting in the channel are performed widely in the low molecules. To use a single crystal in the channel is the most desirable because influence of the grain boundaries can be eliminated. In the case of the polymers, by extending the polymers as much as possible in the direction parallel to the carriers flowing in the channel, the carriers scattering in the polymers are reduced in general.
In order to realize a display device with flexibility, in addition to the TFT for the pixel driving switch, it is necessary to make a peripheral circuit also has flexibility. A TFT used in a pixel driving circuit having larger carrier mobility than about 1 (cm2/V·s) is required. However, the organic TFT that has proved to satisfy this requirement in the present condition is only the TFT in which organic molecules with a small molecular weight is used in the channel. For example, in Science, Vol. 303, p. 1644 (2004), a carrier mobility of 15 (cm2/V·s) is obtained in the organic TFT using a single crystal of an organic molecule, rubrene, in the channel. Further, in Applied Physics Letters, 84, 3061 (2004), a carrier mobility of 35 (cm2/V·s) at room temperature is reported for a single crystal of the highly purified organic molecule, pentacene. However, such high mobility is for a single crystal sample, and is obtained by paying special attention to refining of organic material as raw material, single crystal growth, and manufacture of the TFT. With thin film crystals of organic molecules formed with vacuum deposition that is a general method in the case of using low molecules in the channel, it is difficult to form a single crystal in the channel. Further, because vacuum equipment is used in the manufacture of the TFTs, it is disadvantageous in the aspect of cost and mass production.
In such a way, the organic TFTs have a problem that the performance required from the application side and cost and mass production required in the aspect of production cannot be fulfilled at the same time. That is, a vacuum deposition process is generally used in the manufacture of the organic TFTs consisting of low molecules in which the performance of the TFTs can be easily improved, and it is disadvantageous in the aspect of manufacturing. On the other hand, for the organic TFTs consisting of polymers in which the manufacturing cost can be easily reduced, the performances of the TFTs are extremely low and it is applicable only for limited use.
As a method to solve such problem, there is a method of forming a semiconductor layer of the channel by dissolving low molecules into a solvent and coating. Examples of pentacene that is the most typical organic molecule as an application example to the TFTs of low molecules are disclosed in Journal of Applied Physics, Vol. 79, p. 2136 (1996) and Journal of American Chemical Society, Vol. 124, p. 8812 (2002), in which technology is reported of synthesizing a precursor of a pentacene molecule and forming a thin film using a solution in which solubility to a solvent is improved. Further, in Synthetic Metals, Vol. 153, p. 1 (2005), there is a description about technology of forming a thin film by dissolving pentacene molecules directly into solvents and coating. Furthermore, also in Applied Physics Letters, Vol. 84, p. 3061 (2004) and Japanese Journal of Applied Physics, Vol. 43, No. 2B, p. L315 (2004), there is a description about a procedure to dissolve pentacene molecules into an organic solvent. With these technologies, a low molecular organic thin film can be formed by coating without using vacuum equipment, and the possibility of realizing the required performance with a low cost arises.
The problem is described briefly as follows.
FIG. 1A shows a plan view of the field effect transistor according to an embodiment of the present invention. A lyophobic region 14 is provided on a substrate 16, a source 151 and a drain 152 are provided inside of it, a part to be a channel (a lyophilic region) 12 is provided between them, and by supplying a solution containing semiconductor organic molecules on region 12 and drying it, a channel is formed.
In this embodiment, a recess part 20 is provided.
Examined by the inventors, it was found that the drying direction of the solution cannot be controlled in the case that the recess 20 is not provided (the boundary between the source 151 and the region 12 is made to be linear).
Because of this, a characteristic variation among the TFTs occurs. Therefore, there is a problem that it is difficult to improve not only the device characteristics of the TFTs and uniformity of characteristics among the TFT devices, but also its reliability.
However, not all problems are solved with the above-described coating technology. That is, in the case of forming the channel semiconductor layer of the TFT by coating, it is necessary to limit the region where the semiconductor is formed to the channel. However, only the conventional technology described in Journal of Applied Physics, Vol. 79, p. 2136 (1996), Journal of American Chemical Society, Vol. 124, p. 8812 (2002), and Synthetic Metals, Vol. 153, p. 1 (2005) cannot satisfy the necessity. The technology using a lyophilic and lyophobic pattern as in Japanese Patent Application Laid-Open Publication No. 2004-119479 for example can be applied as a method of limiting the semiconductor layer forming region. Using this technology, the lyophilic and lyophobic pattern is formed on the surface where the solution is being dropped, and the semiconductor layer forming region can be limited by confining the solution within the lyophilic pattern. However, such a technology cannot solve following problems. That is, even though the dropped solution is limited in the lyophilic pattern, the drying direction of the solution cannot be controlled. Because of this, the semiconductor crystals produced after drying are oriented randomly in each channel. Therefore, characteristic variations among TFTs are generated. Depending on the case, for example in the case where the semiconductor concentration of the solution is low, semiconductor films are formed only on a part inside the lyophilic patterns, and there is a possibility that its positions are distributed randomly among the TFTs. The cause of this problem is that at which part of the lyophilic pattern the dropped solution starts to dry, to which direction the drying proceeds, and in which position it dries finally cannot be controlled. That is, this is because the position of the organic semiconductor crystal film formed after drying is determined randomly in each TFT. This exhibits more obviously in the case of increasing the drying speed by heating the substrate and in the case of heating the solution to increase solubility. Furthermore, the size and the orientation of the semiconductor crystal grains formed cannot be controlled. Therefore, it is difficult to improve not only the device characteristics of the TFTs and uniformity of characteristics among the TFT devices, but also reliability.
The present invention is made in view of the above-described problems.